The carbon rods are underneath the maple. Mathias (the luthier) would have preferred to just use just maple and as much of the original mahogany as possible, but in this case he thought the neck would need all the strength it can get because even the initial, badly repaired break was extremely ugly with wood splinters having been glued together in desperation. He shaved all that off - there is only just a thin slice of mahogany remaining where the maple and carbon was added.
Mathias first repaired a neck-broken bass for me more than 20 years ago. That repair was one of his early ones and not exactly pretty or inconspicious, but it lasts to this day. Since then his woodworking skills have improved in leaps and bounds and all his newer repair work is just as lasting and also invisible from the outside.
I still owe you guys a pic - haven't forgotten it, just have to lend a camera here in the office!
Uwe